1972 Super Bass (1992) NO SOUND??

Discussion in 'Let's Talk Vintage' started by recycledsound, Oct 17, 2019.

  1. recycledsound

    recycledsound Member

    Joined:
    Jul 13, 2010
    Messages:
    52
    Likes Received:
    4
    Friend brought this to me saying it was the classis "Tubes Light - No Sound"

    He was sort of right?! Once you turn the volume up to half or more you get an awful distortion crackly sound.... if you hit the strings lightly, you get mostly nothing.

    SO far I just checked the board for obvious issues and while running a limiter I checked to see if I could catch any DC passing the board caps.... didn't disconnect any.

    Anyone seen this problem??

    Thoughts?
     
    ampmadscientist likes this.
  2. ampmadscientist

    ampmadscientist Well-Known Member

    Joined:
    Jun 26, 2014
    Messages:
    13,872
    Likes Received:
    7,706
    Location:
    Start the reactor... Free Mars!
    Buy 1 new 12AX7 tube.
    Replace the preamp tubes one at a time, swap w/ the new one.
    Retest the amp each time.
    If there is a bad preamp tube you will find it.
    This is the most likely cause: a bad preamp tube.
     
    Adieu likes this.
  3. mickeydg5

    mickeydg5 Well-Known Member VIP Member

    Joined:
    Sep 30, 2011
    Messages:
    22,402
    Likes Received:
    9,792
    Location:
    US of A
    It could be quite a few things.
    If capable, measuring the the AC and DC voltages around the power tube sockets may provide clues.
    Are you an electronics technician or know how to work in tube amplifiers with high voltages?
     
  4. mickeydg5

    mickeydg5 Well-Known Member VIP Member

    Joined:
    Sep 30, 2011
    Messages:
    22,402
    Likes Received:
    9,792
    Location:
    US of A
    Ampmadscientist is right about the first thing to do. I would kind of hope however that anyone dealing with vacuum tube devices would think about checking for bad tubes first. Did you or the owner try that yet?
     
  5. South Park

    South Park Well-Known Member

    Joined:
    Sep 14, 2019
    Messages:
    708
    Likes Received:
    440
    Let’s do some trouble shouting . If you get some sound that means the out put is working.Try to tap on the preamp tubes with a pencil to find the bad one
     
  6. recycledsound

    recycledsound Member

    Joined:
    Jul 13, 2010
    Messages:
    52
    Likes Received:
    4
    I use an Orange Tube tester for quick checks.... all tubes passed. I also have a B&K 747 and a Tv2.

    I suspected a volume pot issue but I get a good sweep on both. I think it odd you basically get nothing until you crank it up or really smack the strings, but then you don’t get anything musical, just this crackled whacked out distortion. Like the worst kind of blocking distortion you’ve ever heard.
    Hadn’t seen this exact thing before, so I thought I’d throw it up here just in case some of you might’ve.
     
    mickeydg5 likes this.
  7. Matthews Guitars

    Matthews Guitars Well-Known Member

    Joined:
    Aug 17, 2019
    Messages:
    507
    Likes Received:
    637
    One channel or both?

    I'd suspect a shorted capacitor if the tubes all check out.

    Though not the most probable issue, it MIGHT be a bad output transformer, too. Ohm it out with the power off and caps discharged.

    Has this amp been recapped? Being a 72, it should have certainly been done a number of years ago. No way the original caps would be doing a good job at this point even if they were still (barely) functional.
     
  8. South Park

    South Park Well-Known Member

    Joined:
    Sep 14, 2019
    Messages:
    708
    Likes Received:
    440
    Try to clean the tube sockets and jacks
     
    neikeel likes this.
  9. neikeel

    neikeel Well-Known Member

    Joined:
    Feb 12, 2014
    Messages:
    3,450
    Likes Received:
    1,893
    I kind of take it as read that if you have non-working amp the first thing you do is give it a thorough clean (esp. sockets and output selector switch) clean and lube the pots and install known good valves (not your best set though). I also do static tests on all resistors to check none horribly out of spec (nb some will not read true in PI tail/NFB circuit) check for physically leaking filter caps.
    I would typically bring up to voltage slowly with tubes out if it has been sitting idle, but you don't say what its history is and we don't know if it is stock, modded etc.
    Have you put a clean sinewave (say 1kHZ) in the front end and traced it through the preamp?
    I have seen this before and it was a partial incomplete short in the OT..........
     
  10. South Park

    South Park Well-Known Member

    Joined:
    Sep 14, 2019
    Messages:
    708
    Likes Received:
    440
    It is good to start with basics first. You can go further into the amp
     
  11. ampmadscientist

    ampmadscientist Well-Known Member

    Joined:
    Jun 26, 2014
    Messages:
    13,872
    Likes Received:
    7,706
    Location:
    Start the reactor... Free Mars!
    Set meter for DC volts.
    Connect black lead to metal chassis.
    Red lead connect to output tube sockets. (check voltages)
    Tubes all out. Do not do this with the tubes installed.
    Power On, standby set to operate position.

    Note: these readings are high voltage. The voltage will be approximately 500-600 volts DC.
    Be careful when you measure the tube socket voltages.

    Except for: pin 5 of the output tube sockets, which will be a negative DC voltage less than 100 volts DC.

    1. Test DC voltage on pins 3,4, and 5 of output tube sockets.
    Write down the readings of all 4 sockets and report the voltages to the forum.

    2. Test DC voltage on pins 1 and 6 of phase inverter socket. Report the readings. (this is the preamp tube socket closest to the power tubes)

    3. Test DC voltage on pins 1 and 6 of the middle preamp tube, write down the readings and report the readings.

    4. Test DC voltage on pins 1 and 6 of the 1st preamp tube, the tube closest to the input jack.
    Report the readings.

    Now let's see what all your readings are: (make sure you report ALL the voltages I asked for)

    Power off, unplug mains power. Prepare for Ohm meter readings, next tests.
     
  12. ampmadscientist

    ampmadscientist Well-Known Member

    Joined:
    Jun 26, 2014
    Messages:
    13,872
    Likes Received:
    7,706
    Location:
    Start the reactor... Free Mars!
    Tube testers do not detect all problems.
    Buy 1 new 12AX7 tube and do the tube swap out tests as above.
    Don't assume the tubes are good until you have actually swapped them out 1 at a time with a new tube and retested the amp. (as above)
     
  13. junk notes

    junk notes Active Member

    Joined:
    Dec 10, 2018
    Messages:
    152
    Likes Received:
    134
    1992's rock!

    Yes,
    I had a similar issue, and the Hickok said it was O.K. or 'good'. Did not read 'bad'
    I swapped it out with a NOS valve without issue.
     
  14. mickeydg5

    mickeydg5 Well-Known Member VIP Member

    Joined:
    Sep 30, 2011
    Messages:
    22,402
    Likes Received:
    9,792
    Location:
    US of A
    Is this not akin to my first post?
    Thank you for reiterating. :)
     
  15. mickeydg5

    mickeydg5 Well-Known Member VIP Member

    Joined:
    Sep 30, 2011
    Messages:
    22,402
    Likes Received:
    9,792
    Location:
    US of A
    I would think that he uses his tube testers on other tubes so knows whether or not his equipment is reliable enough to test them.
     
    recycledsound likes this.
  16. mickeydg5

    mickeydg5 Well-Known Member VIP Member

    Joined:
    Sep 30, 2011
    Messages:
    22,402
    Likes Received:
    9,792
    Location:
    US of A
    My first post was because I figured he got so far and it most likely is a power supply issue or connection thereof. That is why we measure voltages around the power tubes in order to figure out proper supply voltages and connections around there. These can give clues as to what may be happening.
     
  17. recycledsound

    recycledsound Member

    Joined:
    Jul 13, 2010
    Messages:
    52
    Likes Received:
    4

    I think it is the OT! Just resistance checking CT to each side of the primary I get 16 ohms on one side, 17 ohms on the other! Close enough to SHORT for me!!
     
  18. Matthews Guitars

    Matthews Guitars Well-Known Member

    Joined:
    Aug 17, 2019
    Messages:
    507
    Likes Received:
    637
    If you had those values on the secondary side, I would think it's OK. but on the primary side, yeah, that sure doesn't seem right at all.
     
    recycledsound likes this.
  19. ampmadscientist

    ampmadscientist Well-Known Member

    Joined:
    Jun 26, 2014
    Messages:
    13,872
    Likes Received:
    7,706
    Location:
    Start the reactor... Free Mars!
    You should do the voltage readings above and report the results.

    Any testing of the OT, remove the tubes first.
     
  20. ampmadscientist

    ampmadscientist Well-Known Member

    Joined:
    Jun 26, 2014
    Messages:
    13,872
    Likes Received:
    7,706
    Location:
    Start the reactor... Free Mars!
    I need to check that ....it may be OK.
     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice